The invention relates to a control arrangement for a plastics extruder including a melt index measuring device consisting of a metering pump whose rotational speed is controllable and which is provided with a rotational speed sensor, a measuring nozzle disposed in a sample flow channel; and pressure and temperature sensors connected to the sample flow channel directly upstream of the inlet of the measuring nozzle and downstream of a screen of the plastics extruder. The invention relates also to a method of regulating the viscosity of molten plastics moulding material by acting on a material flow throttle point following kneading treatment of the plastics material.
Apparatus of the kind mentioned above is known from the house publication F. H. McGinnis, Continuous Capillary Rheometers for On-Line Process Control, SEISCOR Division, Seismograph Service Corporation, Tulsa, Okla. Apparatus of this type was developed with a view to determining, during plastics processing, melt indices such as are conventionally obtained after sampling by standardized laboratory treatment. For this purpose, in the arrangements described in the above-noted publication a sample flow is diverted from the current production process by way of a sample flow channel and, in an adapter channel, is cooled or heated by means of heat exchangers, with an adequate residence time, to the temperature corresponding to the standard temperature for comparison measurements on a laboratory scale, because the comparison data for materials which are customarily used in the plastics industry is adjusted to these standard conditions for laboratory measuring arrangements; see the parameters for melt index measurement which are fixed in ASTM (American Society for Testing Materials Standard) D 1238-65 T or DIN (German Standard Specification) Specification 53735.
From the Federal Republic of Germany Auslegeschrift (Published Patent Application) No. 20 54 615 a control circuit is known which is to achieve coupled regulation of the temperature and pressure of molten plastics material upstream of a moulding tool following a plastics screw press. The controlled port of the system is the channel upstream of the moulding station, while the correcting element used is an annular gap throttle disposed downstream of the screw press and formed between a cylindrical extension of the press screw itself, located inside a hollow cylinder. For the purpose of varying the throttle action, the entire press screw, and consequently also its cylindrical end, is displaced axially in order to vary the effective axial length and hence the flow-resistance of the annular gap. In addition, action on the controlled part of the system is also taken by varying the working rotational speed of the press screw. For the determination of actual values use is made of pressure and temperature sensors upstream of the moulding tool. The comparison of desired and actual values is made by means of a process control computer which is programmed with a response function gradient of the particular screw press, this gradient being recorded at the desired operation point, under operating conditions. This programming is not described herein in detail. It is simply stated that in order to achieve good mouldability of the plastics material the simultaneous regulation of temperature and pressure upstream of the mould tool is desirable and that for this purpose disturbing influences in respect of these variables should be corrected in a very short time. It is, of course, hereby not forgotten that, contrary to the basic assumptions in German Auslegeschrift No. 20 54 615, the pressure upstream of the moulding tool is by no means dependent solely on the rotational speed of the screw, even with a constant throttle gap length. On the contrary, the temperature of the material depends on the gap length, the rotational speed of the screw, and other influencing variables (such as quantity flow, substance values, and effective pressure totals in the plant upstream of the moulding tool), so that the desired controller function cannot be achieved for practical reasons. Apart from quantity flow, substance values, and temperature of the stock, the actual pressure upstream of the moulding tool is in fact dependent primarily on the geometry of the moulding tool. These conditions are described in E. Bernhardt, Process of Thermoplastic Materials, 1959, particularly equation 152 on page 257 and FIG. 4.31 on page 209.
In contrast to the known systems the invention is based on the realisation that on the one hand, in the processing of plastics materials in single-shaft or multishaft screw kneaders the thermal history and consequently the melt viscosity of the molten plastics material which is to be moulded, are very important for the quality of the end product, and that, on the other hand, the known arrangements for on-line sampling (which involves the judging of the probable quality of the moulded end product on the basis of material properties existing upstream of the moulding tool) cannot be used directly because the action of these measuring devices is based on a temperature reaction in the sample taken off and consequently on a comparatively long period of time for making the measurement. Upstream of the moulding tool, however, comparatively rapid fluctuations of the properties of the material, (which can be defined by the instantaneous melt index), may occur, for which reason previously known arrangements appear to be satisfactory only where because of the specific material processing steps such rapid fluctuations are not or are not yet of importance or occur only sufficiently slowly. On the other hand, for certain fields of application values of melt viscosity which lie within fixed limits must be complied with by the molten plastics material which is to be moulded. Thus, particularly in the case of polyolefines the observance of these limits is very decisively dependent on the temperature of the plastics material after it leaves the screw kneader. This temperature is, however determined not only in accordance with the material volume throughput, rotational speed of the screw, the geometry of the screw, and the maintenance of the temperature of the screw casing, but is also very decisively dependent on the pressure of the molten material upstream of the screw tip. In this region the energy dissipated by the screw is directly proportional to pressure, and almost the entire amount of the dissipated energy in converted into heat, thus leading to a corresponding rise of temperature in the pressed plastics material. This increase in temperature caused by the pressure upstream of the screw tip may be desirable, for example in the thermal degradation of polypropylene (namely in the case of a heavy reduction of viscosity). On the other hand, the effect in the normal granulation of polypropylene or of low-pressure polyethylene is undesirable. Before the granulation of such materials, particularly if the granulate is to be of film quality, the molten plastics material is filtered through fine-mesh screens, which gradually clog and are regularly replaced by means of a screen changing device. For the judgement of the quality of the plastics material to be moulded it must therefore be borne in mind that the pressure which has to be applied for passage through the screens must be added to the back-pressure caused by the moulding tool; and this additional pressure is not a constant magnitude but increases with the degree of soiling of the screen, while immediately after a screen is changed the additional pressure drops abruptly to a low initial value. Experiments have shown that pressure increases of between 20 and 40 bars may occur as the result of the clogging of the screens.
The pressure increases which occur because of the clogging of the screens can be compensated by disposing between the outlet end of the screw kneader and the screen changing device a throttle which has a variable passage cross-section for the molten material and whose throttle action is as far as possible reduced in proportion as a pressure rise through the screen pack occurs, in order to ensure that upstream of the moulding tool, the extrusion temperature is kept as constant as possible. However, the control of the throttle cannot be limited to dependence on the instantaneous pressure of the molten material; as previously mentioned the melt index of the plastics material which is to be moulded is in fact of decisive importance for the quality of the material, and the relationship between the viscosity of the molten material and the pressure in the molten material is dependent not only on the material itself, but also upon processing influences resulting from the screw kneader actually used.